Electrocardiography (ECG) is the practice of detecting the electrical activity of the human heart through electrodes. These electrodes are attached to the skin, and their signals are relayed to an ECG machine through leads that are bound into a trunk cable. The leads are made of a conductor, generally tinned copper, surrounded by an insulator, which may be polypropylene. These are then collected together into a trunk that is then provided with a shield, generally made of helically wrapped or woven tinned copper, and a surrounding insulating jacket, which can be polyurethane. So that the cable can be flexible, and to avoid any contact damage between shield and leads, the leads are typically held loosely.
The signal received by the electrodes is very faint, typically in the millivolt range, so it is critically important to minimize noise. The shield prevents outside electromagnetic interference, but internal triboelectric noise remains a problem. There is often movement in the cable due to the patient's breathing, and with the existing design, it is difficult to minimize the buildup of triboelectric charge caused by the rubbing of the loosely bound leads. If this charge builds to a level above the arc point, it will discharge rapidly and create an induced electrical signal that will appear as interfering noise in the conductors.
The shield is helpful in this design because it allows the triboelectric charge caused by the insulated wires rubbing together to be bled away. It is desirable to tightly bind the wires to restrict their movement and reduce triboelectric noise, but a tight polymer wrap would block any triboelectric charges created from reaching the shield, where they could be bled away. This could worsen the effects of triboelectric noise.